Although much is known about the control of both cardiac output and pulmonary ventilation, the mechanisms which precisely match both responses to the metabolic demands of exercise remain incompletely understood. To obtain further insight into this question, this project is designed to study the degree of cardiopulmonary control coupling associated with an exogenous "metabolic" gas load (i.e., appropriately matched CO2 addition and O2 removal) imposed on the systemic circulation of dogs. The question to be answered is whether or not such a load alone can provide the signal for its own normal homeostasis. Because of steady state "plant" constraints on the cardiopulmonary gas transport system, specification of a particular mixed venous composition at each load is sufficient to determine the required values of both cardiac output and ventilation. We wish to determine whether or not the controller can in fact choose these values with no additional information.